In microwave ovens, an enclosure that is generally impermeable to microwave energy forms a cavity within which a body is to be heated, microwave energy is introduced into the cavity, and passage of the microwave energy through the body will tend to vibrate moisture particles and thus create heat within the body. The microwave energy that enters the cavity will enter in a specific pattern and generally be reflected statically throughout the cavity in a pattern that may produce hot and cold spots within a body to be heated. To avoid such a pattern within the cavity, electronic or mechanical mixers will randomly distribute the microwave energy, but still in a somewhat predictable pattern. Various attempts have been made to control the distribution of the microwave energy, for example by motor driven fanlike mixers that will first receive the microwave energy introduced into the cavity to distribute it throughout the cavity, various complicated driven polarizing screens as for example in U.S. Pat. No. 3,189,722 to Fritz of June 15, 1965, or fixed protuberances on the cavity walls for reflecting microwave energy at different angles as for example in the U.S. Pat. No. 3,461,260 to Bremer of Aug. 12, 1969.
Once the microwave energy has achieved the right angle to enter the food, it will usually be completely absorbed and converted to heat, when the oven is used for heating food. An uneven pattern of distribution within the cavity results in certain portions of the food being overheated with other portions of the food being too cool, producing uneven baking or cooking results. It is possible that many of the microwaves will utilize only one stationary wall as a reflecting portion before entering the food, making any stationary protuberances critical on such a wall, since a static entrance of microwaves into the cavity would, upon hitting such a wall, always diffuse in the same pattern. Only by movement of such a wall or other mixer can the undesirable effect of uneven distribution be minimized by creating an increased diffusion.
As in conventional ovens, it is well known to provide an area of visibility or a window through the enclosure of the oven for viewing of the body being heated, particularly the food being cooked. However, such a window or area of visibility must be shielded with respect to microwave energy, so that it will not pass microwave energy from the cavity to the exterior of the enclosure. Such shielding usually employs a plurality of closely spaced aperatures in a metal shield, with the dimensions of the aperatures being small enough to prevent passage of the microwave energy. However, such shields with their small aperatures greatly decrease the visibility through the window and therefore it is difficult to visually determine the precise appearance of foods during preparation, such as the degree of cooking, liquid starting to boil, and the like, which can result in overcooking the food, creation of a mess from boiling within the oven, uneven cooking due to the cook's failure to reposition foods should such a need be visually indicated by uneven cooking patterns during the cooking process, and a careful user must open and close the oven door quite frequently to obtain a better visual check on the food being prepared, which would result in starting and stopping the unit frequently, which can shorten the life of the magnetron tube.
Even though it may be possible to employ a special type of glass in the door, there is always the danger of a break or crack, so that still the microwave shield is still the major consumer protection against direct leakage of the microwave through a window area, and their necessary construction will greatly decrease the visibility through the window. This decrease in visibility is due not only to the small effective open area but also due to the fact that the shield is in a separate plane spaced from the food which will represent a distraction to the eyes attempting to focus beyond the plane of the shield.